/Engine/Plugins/FX/Niagara/Source/Niagara/Public/NiagaraDataChannelAccessor.h
Writing: Ideally write once per frame. To write, construct an FScopedWriter from yourFNiagaraDataChannelGameDataGroupedWriter. This new scoped writer will allocate space to write as many items as have been added in each group via AddItem. Writing more than this will not cause any major harm but will cause additional allocations. Results will be published to the NDC when the scoped writer is destroyed. Call Append(), passing each item's GroupIndex and the data to write.
struct FNiagaraDataChannelGameDataGroupedWriterBase